Dishwasher manufacturers have a need for accurately determining the water fill volume of each wash and rinse cycle. However, because the water pressure varies from installation location to location, the fill rate, i.e. the amount of water that flows for a given period of time, varies. As such, a timed fill cycle may result in too much or too little water being dispensed. To properly operate the appliance, therefore, it is important to actually measure the amount of water that is dispensed during the fill cycle. This can be determined by either a level switch that senses the amount of water in the tub, or by sensing the water as it is being dispensed through some type of flow measuring device.
A turbine-type flow device with magnetic sensing has been used in the past to measure the flow of water through the flow control valve during the water dispensing period. The controller of the appliance detects the electrical pulses generated as the turbine rotates under the flow of the water, and calculates the amount of water that has been dispensed. Once the proper amount of water has been dispensed, as determined by the appliance controller, the flow control valve is turned off. One such turbine-type flow metering device is described in U.S. Pat. No. 5,429,272, entitled Device for Controlling, by Means of an Electrovalve, the Volume Liquid Flowing to a Receptacle.
Unfortunately, there are limitations on the accuracy of such turbine-type flow metering devices that limit the ability of the controller to precisely determine how much water has been dispensed by the valve. Such limitations are due, at least in part, to the lack of efficiency of the energy transfer from the water to make the turbine rotate. Such force from the flowing water stream must overcome the friction and magnetic interference that tends to impede the rotation of the turbine. Such problems are particularly acute in installations that have a low water pressure, and therefore a low flow rate of the water through the valve and over the turbine-type flow meter. At such low flow rates, the turbine tends to rotate intermittently or at a rate that does not truly reflect the amount of water that has been dispensed.
There is, therefore, a need in the art for a fluid flow metering device that increases the efficiency of a standard turbine-type flow metering device with magnetic sensing. The present invention provides such a device. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.